Apparatus and method for performing uplink synchronization in wireless communication system

ABSTRACT

This specification relates to an apparatus and method for performing random access in a wireless communication system. This specification discloses a mobile station, including a reception unit for receiving TAG configuration information on which at least one serving cell configured in the mobile station is classified as a Timing Alignment Group (TAG) from a base station and a transmission unit for transmitting a random access preamble to the base station on one representative serving cell within the TAG. In accordance with this specification, a procedure of obtaining a TAV for a serving cell in order to secure and maintain uplink timing synchronization becomes clear, the time taken to obtain uplink synchronization for a serving cell may be reduced, and overhead due to excessive random access attempts may be reduced by obtaining a TAV for a plurality of serving cells through one random access procedure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from and the benefit of Korean PatentApplication Nos. 10-2011-0050811 filed on May 27, 2011, 10-2011-0052957filed on Jun. 1, 2011, 10-2011-0111531 filed on Oct. 28, 2011,10-2011-0116671 filed on Nov. 9, 2011 and 10-2012-0020621 filed on Feb.28, 2012, which are all incorporated herein by reference for allpurposes as if fully set forth herein.

BACKGROUND

1. Field of the Invention

The present invention relates to wireless communication and, moreparticularly, to an apparatus and method for performing uplinksynchronization in a wireless communication system.

2. Discussion of the Background

In a common wireless communication system, although an uplink bandwidthand a downlink bandwidth are different, only one carrier is chieflytaken into consideration. Even in 3rd Generation Partnership Project(3GPP) Long Term Evolution (LTE), the number of carriers forming uplinkand downlink is 1, and an uplink bandwidth and a downlink bandwidth arecommonly symmetrical to each other on the basis of a single carrier. Amultiple carrier system is recently introduced.

A multiple carrier system means a wireless communication system capableof supporting a carrier aggregation (CA). The carrier aggregation istechnology for efficiently using fragmented and small bands, and it hasan effect that a logically large band is used by binding a plurality ofbands which are physically non-contiguous to each other in the frequencyregion.

In order for a mobile station to access a network, the mobile stationmust perform a random access procedure. The random access procedure maybe divided into a contention-based random access procedure and anon-contention-based random access procedure. The greatest differencebetween the contention-based random access procedure and thenon-contention-based random access procedure lies in whether a randomaccess preamble is dedicated and designated to one mobile station. Inthe non-contention-based random access procedure, contention (or acollision) between mobile stations does not occur because a mobilestation uses a random access preamble dedicated thereto itself. Here,the term ‘contention’ means that two or more mobile stations attempt arandom access procedure using the same random access preamble throughthe same resources. In the contention-based random access procedure,there is a possibility of contention because a mobile station uses arandomly selected random access preamble.

Objects of a mobile station to perform a random access procedure for anetwork may include initial access, handover, a scheduling request,timing alignment, and so on.

A UE has performed a random access using one carrier in a conventionalsingle carrier system. But with regards to current CA support, the UE isable to support the random access with multiple component carriers.There is a need for the detailed random access procedure of a UE inmultiple component carrier system.

SUMMARY

An object of the present invention is to provide a method and apparatusfor performing uplink synchronization in a wireless communicationsystem.

Another object of the present invention is to provide an apparatus andmethod for configuring a message including a group for performing uplinksynchronization and a timing alignment value (TAV) in a wirelesscommunication system.

Yet another object of the present invention is to provide an apparatusand method for transmitting a TAV which is applied to a plurality ofSSCs in common.

Further yet another object of the present invention is to provide anapparatus and method for obtaining a TAV which is applied to a pluralityof SSCs in common through a single message.

Further yet another object of the present invention is to provide anapparatus and method for transmitting a Medium Access Control (MAC)message including a TAV for each SSC group.

Further yet another object of the present invention is to provide anapparatus and method for classifying a group including SSCs to which thesame TAV is applied.

Further yet another object of the present invention is to provide anapparatus and method for generating a message indicating a groupincluding a SSC to which the same TAV is applied.

Further yet another object of the present invention is to provide anapparatus and method for checking a TAV for each group including SSCbased on a MAC message.

In accordance with an aspect of the present invention, there is provideda mobile station, including a reception unit for receiving TAGconfiguration information on which at least one serving cell configuredin the mobile station is classified as a Timing Alignment Group (TAG)from a base station and a transmission unit for transmitting a randomaccess preamble to the base station on one representative serving cellwithin the TAG.

The reception unit receives a random access response message, includinga Timing Advance Command (TAC) field from the base station in responseto the random access preamble, and the TAC field indicates a TAV onwhich the uplink timing of all the serving cells within the TAG isidentically adjusted.

In accordance with another aspect of the present invention, there isprovided a method of a mobile station performing random access in awireless communication system. The method of performing random accessincludes receiving TAG configuration information on which at least oneserving cell configured in the mobile station is classified as a TAGfrom a base station, transmitting a random access preamble to the basestation on one representative serving cell within the TAG, and receivinga random access response message, including a TAC field, from the basestation in response to the random access preamble.

The TAC field indicates a TAV on which the uplink timing of all theserving cells within the TAG is identically adjusted.

In accordance with yet another aspect of the present invention, there isprovided a base station performing random access in a wirelesscommunication system. The base station includes a Radio Resource Control(RRC) processing unit for generating TAG configuration information onwhich at least one serving cell configured in a mobile station isclassified as a TAG, a transmission unit for transmitting the TAGconfiguration information to the mobile station, a reception unit forreceiving a random access preamble from the mobile station on onerepresentative serving cell within the TAG, a random access processingunit for generating a random access response message including a TACfield indicating a TAV on which the uplink timing of all the servingcells within the TAG is identically adjusted in response to the randomaccess preamble, and a transmission unit for transmitting the randomaccess response message to the mobile station.

In accordance with further yet another aspect of the present invention,there is provided a method of a base station performing random access ina wireless communication system. The method of performing random accessincludes transmitting TAG configuration information on which at leastone serving cell configured in a mobile station is classified as a TAGto the mobile station, receiving a random access preamble from themobile station on one representative serving cell within the TAG, andtransmitting a random access response message, including a TAC fieldindicating a TAV on which the uplink timing of all serving cells withinthe TAG is identically adjusted to the mobile station in response to therandom access preamble.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompany drawings, which are included to provide a furtherunderstanding of this document and are incorporated on and constitute apart of this specification illustrate embodiments of this document andtogether with the description serve to explain the principles of thisdocument.

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 shows an example of a protocol structure for supporting multiplecarriers to which the present invention is applied.

FIG. 3 shows an example of a frame format for a multiple carrieroperation to which the present invention is applied.

FIG. 4 shows a simple concept of a multiple carrier system.

FIG. 5 is a flowchart illustrating a method of transmitting TAGconfiguration information according to an example of the presentinvention.

FIG. 6 is a flowchart illustrating a method of transmitting TAGconfiguration information according to another example of the presentinvention.

FIG. 7 is a flowchart illustrating a method of transmitting TAGconfiguration information according to yet another example of thepresent invention.

FIG. 8 is a flowchart illustrating a method of transmitting TAGconfiguration information according to further yet another example ofthe present invention.

FIG. 9 shows an example of a MAC sub-header according to an example ofthe present invention;

FIG. 10 is a diagram showing an MAC CE for a TAG according to an exampleof the present invention.

FIG. 11 is a diagram showing an MAC CE for a TAG according to anotherexample of the present invention.

FIG. 12 is a flowchart illustrating for reception of Timing AlignmentCommand (TAC) according to an example of the present invention.

FIG. 13 shows an example in which DCI is mapped to an EPDCCH accordingto the present invention.

FIG. 14 shows another example in which DCI is mapped to an ExtendedPDCCH according to the present invention.

FIG. 15 shows yet another example in which DCI is mapped to an extendedphysical downlink control channel according to the present invention.

FIG. 16 is a flowchart illustrating for reception of Timing AlignmentCommand (TAC) according to another example of the present invention.

FIG. 17 shows a Medium Access Control Protocol Data Unit (MAC PDU)format to which the present invention is applied.

FIG. 18 is a block diagram showing a MAC CE for a Timing AlignmentCommand (TAC) according to an example of the present invention.

FIG. 19 is a block diagram showing a MAC CE for a TAC according toanother example of the present invention.

FIG. 20 is a block diagram showing a MAC CE for a TAC according to yetanother example of the present invention.

FIGS. 21 and 22 are block diagrams showing MAC CEs for a TAC accordingto yet another example of the present invention.

FIG. 23 is a flowchart illustrating the operation of a UE which performsuplink synchronization according to an example of the present invention.

FIG. 24 is a flowchart illustrating the operation of a BS which performsuplink synchronization according to an example of the present invention.

FIG. 25 is an explanatory diagram illustrating a method of performinguplink synchronization by using a TAV in a multiple CC system.

FIG. 26 is a block diagram showing an apparatus which perform uplinksynchronization according to an example of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Hereinafter, in this specification, some exemplary embodiments aredescribed in detail with reference to the accompanying drawings. It isto be noted that in assigning reference numerals to elements in thedrawings, the same reference numerals designate the same elementsthroughout the drawings although the elements are shown in differentdrawings. Furthermore, in describing the embodiments of the presentinvention, a detailed description of the known functions andconstructions will be omitted if it is deemed to make the gist of thepresent invention unnecessarily vague.

Furthermore, this specification is focused on a wireless communicationnetwork. Tasks performed in the wireless communication network may beperformed in a process in which a system (e.g., a base station) managingthe wireless communication network controls the wireless communicationnetwork and transmits data or may be performed in a mobile stationassociated with the wireless communication network.

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

Referring to FIG. 1, the wireless communication systems 10 are widelydeployed in order to provide various communication services, such asvoice and packet data. The wireless communication system 10 includes atleast one Base Station (BS) 11. Each BS 11 provides communicationservice to specific cells 15 a, 15 b, and 15 c. Each of the cells may beclassified into a plurality of areas (called sectors).

A user equipment (UE) 12 may be fixed or mobile and may also be calledanother terminology, such as a mobile station (MS), a Mobile Terminal(MT), a User Terminal (UT), a Subscriber Station (SS), a wirelessdevice, a Personal Digital Assistant (PDA), a wireless modem, or ahandheld device. The BS 11 may also be called another terminology, suchas an evolved-NodeB (eNB), a Base Transceiver System (BTS), an accesspoint, a femto BS, a home nodeB, or a relay. The cell should beinterpreted as a comprehensive meaning that indicates some of a regioncovered by the BS 11, and it has a meaning to cover various coverageregions, such as a mega cell, a macro cell, a micro cell, a pico cell,an a femto cell.

Hereinafter, downlink (DL) refers to communication from the BS 11 to theUE 12, and uplink (UL) refers to communication from the UE 12 to the BS11. In downlink, a transmitter may be a part of the BS 11, and areceiver may be a part of the UE 12. In UL, a transmitter may be a partof the UE 12, and a receiver may be a part of the BS 11. Multiple accessschemes applied to the wireless communication system are not limited. Avariety of multiple access schemes, such as Code Division MultipleAccess (CDMA), Time Division Multiple Access (TDMA), Frequency DivisionMultiple Access (FDMA), Orthogonal Frequency Division Multiple Access(OFDMA), Single Carrier-FDMA (SC-FDMA), OFDM-FDMA, OFDM-TDMA, andOFDM-CDMA, may be used. In uplink transmission and downlinktransmission, a Time Division Duplex (TDD) method of performingtransmission using different times may be used or a Frequency DivisionDuplex (FDD) method of performing transmission using differentfrequencies may be used.

A Carrier Aggregation (CA) is to support a plurality of CCs and alsocalled a spectrum aggregation or a bandwidth aggregation. Each ofindividual unit carriers aggregated by a CA is called a ComponentCarrier (hereinafter referred to as a ‘CC’). Each CC is defined by thebandwidth and the center frequency. The CA is introduced in order tosupport an increasing throughput, prevent an increase of costs due tothe introduction of a broadband Radio Frequency (RF) device, andguarantee compatibility with the existing system. For example, assumingthat 5 CCs are allocated as the granularity of a carrier unit having abandwidth of 20 MHz, a maximum bandwidth of 100 MHz can be supported.

The size (i.e., a bandwidth) of CCs may be different. For example, if 5CCs are used in order to configure a band of 70 MHz, the band may have 5MHz CC (carrier #0)+20 MHz CC (carrier #1)+20 MHz CC (carrier #2)+20 MHzCC (carrier #3)+5 MHz CC (carrier #4).

Hereinafter, a multiple CC system refers to a system supporting acarrier aggregation. In a multiple CC system, a contiguous carrieraggregation and/or a non-contiguous carrier aggregation may be used.Furthermore, either a symmetrical aggregation or an asymmetricalaggregation may be used.

FIG. 2 shows an example of a protocol structure for supporting multiplecarriers to which the present invention is applied.

Referring to FIG. 2, a Medium Control Access (MAC) entity 210 manages aphysical layer (PHY) 220 using a plurality of carriers. A MAC managementmessage transmitted through a specific carrier may be applied to othercarriers. That is, the MAC management message is a message which maycontrol other carriers including the specific carrier. The physicallayer 220 may be operated in a Time Division Duplex (TDD) scheme and/orin a Frequency Division Duplex (FDD) scheme.

Some physical control channels are used in the physical layer 220. APhysical Downlink Control Channel (PDCCH) informs a UE of the resourceallocation of a Paging CHannel (PCH) and a downlink shared channel(DL-SCH) and of information about a Hybrid Automatic Repeat Request(HARQ) related to the DL-SCH. The PDCCH may carry an UL grant thatinforms a UE of the resource allocation of uplink transmission. APhysical Control Format Indicator Channel (PCFICH) informs a UE of thenumber of OFDM symbols used in PDCCHs, and it is transmitted persubframe. A Physical Hybrid ARQ Indicator Channel (PHICH) carries anHARQ ACK/NAK signal in response to uplink transmission. A PhysicalUplink Control Channel (PUCCH) carries HARQ ACK/NAK for downlinktransmission, a scheduling request, and UL control information, such asCQI. A Physical Uplink Shared Channel (PUSCH) carries an uplink sharedchannel (UL-SCH). A Physical Random Access Channel (PRACH) carries arandom access preamble.

FIG. 3 shows an example of a frame format for a multiple carrieroperation to which the present invention is applied.

Referring to FIG. 3, a frame includes 10 subframes. Each of thesubframes includes a plurality of OFDM symbols. Each of the carriers maycarry its own control channel (e.g., a PDCCH). Multiple carriers may becontiguous to each other or may not be contiguous to each other. A UEmay support one or more carriers according to its capabilities. Here,physical control format indicator channel (PCFICH) is mapped to thefirst OFDM symbol among the plurality OFDM symbols to indicate a regionon which PDCCH is transmitted through a DL CC.

FIG. 4 shows a simple concept of a multiple carrier system.

Referring to FIG. 4, in an aspect, DL CCs D1, D2, and D3 are aggregatedand uplink CCs (hereinafter referred to as ‘UL CCs’) U1, U2, and U3 areaggregated. Here, Di is the index of a DL CC, and Ui is the index of aUL CC (where i=1, 2, 3). Each of the indices is not always equal to thesequence of CC or frequency band for the corresponding CC.

At least one DL CC may be configured as a PCC, and the remaining CCs maybe configured as SCCs. And at least one UL CC may be configured as aPCC, and the remaining CCs may be configured as SCCs. For example, D1and U1 may be PCCs, and D2, U2, D3, and U3 may be SCCs.

Here, the index of the PCC may be set to 0, and one of natural numberother than 0 may be the index of the SCC. Furthermore, the indices ofDL/UL CCs may be set identically with the indices of CCs (or servingcells) including the relevant DL/UL CCs. For another example, only a CCindex or an SCC index may be set, and UL/DL CC indices included in therelevant CCs may not exist.

In an FDD system, a DL CC and a UL CC may be linked to each other in aone-to-one manner. D1 and U1, D2 and U2, and D3 and U3 may be linked toeach other in a one-to-one manner. A UE sets up linkages between DL CCsand UL CCs on the basis of system information transmitted on a logicalchannel BCCH or a UE-dedicated RRC message transmitted on a DCCH. Eachlinkage may be set up in a cell-specific manner or a UE-specific manner.This linkage is called System Information Block 1 (SIB1) linkage orSystem Information Block 2 (SIB2) linkage. Each linkage may be set up ina cell-specific or UE-specific manner. For example, a PCC may beconfigured in a cell-specific manner, and an SCC may be configured in aUE-specific manner.

Here, not only the 1:1 linkage between the DL CC and the UL CC but alsoa 1:n or n:1 linkage may also be set up or established.

A DL CC corresponding to a PSC is called a DL PCC, and a UL CCcorresponding to a PSC is called a UL PCC. Furthermore, in downlink, aCC corresponding to an SSC is called a DL SCC. In uplink, a CCcorresponding to an SSC is called a UL SCC. Only DL CC may correspond toone serving cell, or both a DL CC and a UL CC may correspond to oneserving cell.

A Primary Serving Cell (PSC) refers to one serving cell which providessecurity input and NAS mobility information in an RRC establishment orre-establishment state. At least cell may be configured to form a set ofserving cells along with a PSC according to the capabilities of a UE.The at least one cell is called a Secondary Serving Cell (SSC).Accordingly, a set of serving cells configured for one UE may includeonly one PSC or may include one PSC and at least one SSC.

As described above, in a multiple CC system, a concept thatcommunication between a UE and a BS is performed through a DL CC or a ULCC is the same as a concept that communication between a UE and a BS isperformed through a serving cell. For example, in a method of performinga random access procedure according to the present invention, a conceptthat a UE transmits a preamble using a UL CC may be considered as thesame concept that a UE transmits a preamble using a PSC or SSC.Furthermore, a concept that a UE receives DL information using a DL CCmay be considered as the same concept that a UE receives DL informationusing a PSC or SSC.

A PSC and an SSC have the following characteristics.

First, a PSC is used for the transmission of a PUCCH. In contrast, anSSC is unable to transmit a PUCCH, but able to transmit some of piecesof control information within a PUCCH through a PUSCH.

Second, a PSC is always activated, whereas an SSC is activated ordeactivated according to a specific condition. The specific conditionmay include that the SSC has received the activation/deactivation MAC CEmessage of an eNB or a deactivation timer within a UE has expired.

Third, when a PSC experiences a Radio Link Failure (RLF), RRCre-establishment is triggered, or when an SSC experiences an RLF, RRCre-establishment is not triggered. An RLF is generated when DLcapabilities equal to or lower than a critical value are maintained fora specific time or higher or an RACH has failed critical times or more.

Fourth, a PSC may be changed by a change of a security key or may bechanged by a handover procedure accompanied by an RACH procedure.However, in case of a Contention Resolution (CR) message, only a PDCCHindicating CR must be transmitted through a PSC, and CR information maybe transmitted through a PSC or SSC.

Fifth, Non-Access Stratum (NAS) information is received through a PSC.

Sixth, a PSC always consists of a pair of a DL PCC and a UL PCC.

Seventh, a different CC may be configured as a PSC for each UE.

Eighth, procedures, such as the reconfiguration, addition, and removalof an SSC, may be performed by a Radio Resource Control (RRC) layer. Inadding a new SSC, RRC signaling may be used to transmit systeminformation about a dedicated SSC.

Ninth, a PSC may provide both a PDCCH (e.g., downlink allocationinformation or uplink grant information) allocated to a UE-specificsearch space which is configured in order to transmit controlinformation to only a specific UE within a region where the controlinformation is transmitted and a PDCCH (e.g., System Information (SI), aRandom Access Response (RAR), and Transmit Power Control (TPC))allocated to a Common Search Space (CSS) which is configured in order totransmit control information to all UEs within a cell or a plurality ofUEs matching with a specific condition. In contrast, an SSC may beconfigured for only a UE-specific search space. That is, since a UEcannot check a CSS through an SSC, the UE is unable to receive pieces ofcontrol information transmitted only through the CSS and pieces of datainformation indicated by the pieces of control information.

From among SSCs, an SSC in which a CSS may be defined may be defined.This SSC is called a special SSC. The SCell is always configured as ascheduling cell at the time of cross-carrier scheduling. Furthermore, aPUCCH configured for a PSC may be defined for an SCell.

The PUCCH for an SCell may be fixedly configured when an SCell isconfigured or may be allocated (configured) or released by an RRCreconfiguration message when a BS reconfigures a relevant SSC.

The PUCCH for the SCell includes ACK/NACK information or Channel QualityInformation (CQI) about SSCs which exist within a relevant sTAG. Asdescribed above, a BS may configure the PUCCH for an SCell through RRCsignaling.

Furthermore, a BS may configure one SCell from among a plurality of SSCsor may not configure an SCell. The reason why a BS does not configure anSCell is that it is determined that a CSS and a PUCCH do not need to beconfigured. For example, if it is determined that a contention-basedrandom access procedure does not need to be performed in any SSC or ifit is determined that the capacity of the PUCCH of a current PSC issufficient, a PUCCH for an additional SSC does not need to beconfigured.

In a wireless communication environment, while electric waves arepropagated from a transmitter and transferred to a receiver, theelectric waves experience propagation delay. Accordingly, although boththe transmitter and the receiver precisely know the time that theelectric waves are propagated from the transmitter, the time that asignal takes to reach the receiver is influenced by the distance betweenthe transmitter and the receiver, a surrounding electric waveenvironment, etc. If the receiver is moving, the time is also changed.If the receiver does not precisely know the time taken for the signal tobe received from the transmitter, the receiver does not receive thesignal, or communication is impossible although the signal is receivedbecause a distorted signal is received.

Accordingly, in a wireless communication system, in order to receive aninformation signal in both downlink/uplink, synchronization must beperformed between a BS and a UE. The type of synchronization includesframe synchronization, information symbol synchronization, samplingperiod synchronization, and so on. Sampling period synchronization mustbe obtained most basically in order to distinguish physical signals fromone another.

DL synchronization is obtained by a UE on the basis of the signal of aBS. The BS transmits an agreed signal so that the UE may easily obtaindownlink synchronization. The UE must be able to precisely know the timewhen a specific signal was sent from the BS. In case of downlink, sinceone BS transmits the same synchronization signal to a plurality of UEsat the same time, the UEs may obtain synchronization independently.

In case of uplink, a BS receives signals transmitted by a plurality ofUEs. If the distance between each UE and each BS is different, a signalreceived by each BS has a different transmission delay time. If uplinkinformation is transmitted on the basis of obtained downlinksynchronization, a relevant BS receives information about each UE on adifferent time. In this case, the BS is unable to obtain synchronizationon the basis of any one UE. Accordingly, in order to obtain uplinksynchronization, a different procedure from that of downlink isnecessary.

Meanwhile, a need to obtain uplink synchronization may be differentaccording to a multiple access scheme. For example, in case of a CDMAsystem, although a BS receives the uplink signals of different UEs ondifferent times, the BS may separate the uplink signals from oneanother. However, in a wireless communication system based on OFDMA orFDMA, a BS demodulates the uplink signals of all UEs at once and at thesame time. Accordingly, reception performance is increased as the uplinksignals of a plurality of UEs are received on precise time, butreception performance is suddenly deteriorated as a difference betweenthe times when the uplink signals of UEs are received is increased.Accordingly, it is indispensable to obtain uplink synchronization.

An exemplary method to obtain or acquire the uplink synchronization (inother words, uplink timing adjustment or uplink timing alignment) is arandom access procedure. During a random access procedure, a UE obtainsuplink synchronization on the basis of a Timing Alignment Value (TAV)transmitted from a BS. A TAV may be called a timing advance value inthat the TAV has a value that advances the uplink timing. Alternatively,a TAV may also be called a timing adjustment value. A random accessprocedure is used to obtain a TAV for the uplink timing synchronizationof an SSC.

If uplink synchronization is obtained, a UE starts a Time AlignmentTimer (TAT). While a TAT is operated, a UE and a BS determine that theyhave been uplink-synchronized. If a TAT expires or a TAT is notoperated, a UE and a BS determine that they have not been synchronizedwith each other, and thus the UE does not perform uplink transmissionother than the transmission of a random access preamble.

Meanwhile, in a multiple carriers system, one UE performs communicationwith a BS through a plurality of CCs or a plurality of serving cells. Ifall signals transmitted from a UE to a BS through a plurality of servingcells have the same time delay, the UE may obtain uplink synchronizationfor all serving cells through only one TAV. In contrast, if signalstransmitted from a UE to a BS through a plurality of serving cells havedifferent time delays, a different TAV is required for each servingcell. In this case, several TAVs may exist. They are called multipleTAVs. Furthermore, an uplink synchronization procedure related tomultiple TAVs is called Multiple Timing Alignment (M-TA) or MultipleTiming Advance (M-TA).

If a UE performs a random access procedure for each serving cell inorder to obtain multiple TAVs, overhead is generated in limited uplinkresources, and the complexity of random access may be increased. Inorder to reduce the overhead and the complexity, a Timing AlignmentGroup (TAG) is defined. A TAG may also be called a timing advance group.And the pTAG indicates a TAG including a PSC, and the sTAG indicates aTAG not including a PSC.

A serving BS and a UE may perform the following operations in order toobtain and maintain a Timing Alignment Value (TAV) for each of TAGs.

1. A serving BS and a UE obtain and maintain the TAV of a pTAG through aPSC. Furthermore, a timing reference, that is, a criterion forcalculating and applying the TAV of a pTAG always becomes a DL CC withina PSC.

2. In order to obtain an initial UL TAV for an sTAG, anon-contention-based RA procedure initialized by a BS is used.

3. One of activated SSCs may be used as a timing reference for an sTAG.However, it is assumed that there is no change of an unnecessary timingreference.

4. Each TAG has one timing reference and one Timing Alignment Timer(TAT). Furthermore, each TAT may include a different timer expirationvalue. A TAT is started or restarted right after a TAV is obtained froma serving BS in order to inform the validity of a TAV that has beenobtained and applied by each TAG.

5. If the TAT of a pTAG is not operated, a TAT for all sTAG should notbe operated. That is, if the TAT of a pTAG expires, the TAT of all TAGsincluding the pTAG expires. If a TAT for a pTAG is not operated, a TATfor all sTAG cannot be started.

A. If the TAT of a pTAG expires, a UE flushes the HARQ buffers of allserving cells. Furthermore, the UE clears a resource allocationconfiguration for all downlink and uplink. For example, likeSemi-Persistent Scheduling (SPS), if periodic resource allocation isconfigured without control information transmitted for the purpose ofresource allocation for downlink/uplink, such as a PDCCH, a UE clears anSPS configuration. Furthermore, a UE releases the configuration of thePUCCHs and type 0 (periodic) SRS of all serving cells.

6. If only the TAT of an sTAG expires, the following procedure isperformed.

A. SRS transmission through the UL CCs of SSCs within an sTAG isstopped.

B. A type 0 (periodic) SRS configuration is released. A type 1(aperiodic) SRS configuration is maintained.

C. Configuration information about a CSI report is maintained.

D. HARQ buffers for uplink of SSCs within an sTAG are flushed.

7. If a TAT for an sTAG is performed, although all SSCs within the sTAGare deactivated, a UE performs the TAT of the sTAG without stopping theTAT of the sTAG. It means that the validity of the TAV of the relevantsTAG can be guaranteed through the TAT although all the SSCs within thesTAG are deactivated and thus a situation in which any SRS and uplinktransmission for tracing uplink synchronization are not performed ismaintained for a specific time.

8. If the last SSC within an sTAG is removed, that is, any SSC is notconfigured within the sTAG, a TAT within the sTAG is stopped.

9. A random access procedure for an SSC may be performed when a BStransmits a PDCCH order, indicating the start of the random accessprocedure, to an activated SSC through a PDCCH, that is, a physicallayer control information channel. The PDCCH order includes randomaccess preamble index information which may be used in an SSC within thesTAG of a relevant UE and PRACH mask index information which permits thetransmission of a random access preamble to all or some oftime/frequency resources available for the relevant SSC. Accordingly,the random access procedure for the SSC is performed only through anon-contention-based random access procedure. Here, in order to indicatethe non-contention-based random access procedure, random access preambleinformation included in the PDCCH order must be indicated by informationother than ‘000000’.

10. A PDCCH and a PDSCH for transmitting a Random Access Response (RAR)message may be transmitted through a PSC.

11. When the number of times that the random access preamble of an SSCis retransmitted reaches a maximum permissible retransmission number: A)An MAC layer stops a random access procedure. B) An MAC layer does notinform an RRC layer that a random access procedure has failed.Accordingly, the triggering of a Radio Link Failure (RLF) is not caused.C) A UE does not inform a BS that the random access procedure of an SSChas failed.

12. The path attenuation reference of a pTAG may become a PSC or an SSCwithin a pTAG, and a BS may differently set the path attenuationreference for each serving cell within a pTAG through RRC signaling.

13. The path attenuation reference of the UL CCs of each of servingcells within an sTAG is an SIB2-linked DL CC. Here, the meaning that thepath attenuation reference has been SIB2-linked means that a linkagebetween a DL CC configured based on information within the SIB1 of arelevant SSC and a UL CC configured based on information within theSIB2. Here, the SIB2 is one of system information blocks transmittedthrough a broadcasting channel, and the SIB2 is transmitted from a BS toa UE through an RRC reconfiguration procedure when an SSC is configured.Uplink center frequency information is included in the SIB2, anddownlink center frequency information is included in the SIB1.

FIG. 5 is a flowchart illustrating a method of transmitting TAGconfiguration information according to an example of the presentinvention.

Referring to FIG. 5, a UE transmits classifying assistant information toa BS at step S500. The classifying assistant information providesinformation or a criterion which is necessary to classify at least oneserving cell, configured in a UE, as a TAG. For example, the classifyingassistant information may include at least one of geographical positioninformation about a UE, neighbor cell measurement information about aUE, network deployment information, and serving cell configurationinformation. The geographical position information of a UE indicates aposition which may be represented by the latitude, altitude, and heightof the UE. The neighbor cell measurement information of a UE includesReference Signal Received Power (RSRP) of a reference signal transmittedfrom a neighbor cell or Reference Signal Received Quality (RSRQ) of areference signal. The network deployment information indicates thedeployment of a BS, a Frequency Selective Repeater (FSR), or a RemoteRadio Head (RRH). The serving cell configuration information isinformation about a serving cell configured for a UE. The step S500indicates that the UE transmits the classifying assistant information tothe BS, but the BS may know the classifying assistant informationadditionally or may already have the classifying assistant information.In this case, a random access procedure according to the presentembodiment may be performed without the step S500.

The BS configures a TAG by classifying serving cells at step S505. Theserving cells may be classified into TAGs or configured as TAGs based onthe classifying assistant information. The TAG may be defined asfollows. The TAG is a group of at least one serving cell, and the sameTAV is applied to serving cells within a TAG. For example, if a firstserving cell and a second serving cell belong to the same TAG TAG1, thesame TAV TA1 is applied to the first serving cell and the second servingcell. In contrast, if the first serving cell and the second serving cellbelong to different TAGs TAG1 and TAG2, different TAVs TA₁ and TA₂ areapplied to the first serving cell and the second serving cell,respectively. The TAG may include a PSC, may include at least one SSC,or may include a PSC and at least one SSC. Alternatively, the TAG may bedefined as a group of serving cell(s) which use the same TAV and thesame timing reference. Alternatively, the TAG may be a group using atiming reference cell including timing reference. Here, the timingreference is a DL CC which is a criterion for calculating the TAV.

For example, a BS may configure a TAG in a UE-specific manner. Servingcell configuration information is configured for each UE individuallyand independently. Thus, if the serving cell configuration informationis used as classifying assistant information, a TAG may be configuredfor each UE individually and independently. For example, it is assumedthat TAGs for a first UE include TAG1_UE1 and TAG2_UE1 and TAGs for asecond UE include TAG1_UE2 and TAG2_UE2. If first and second servingcells are configured for the first UE, TAG1_UE1={the first servingcell}, and TAG2_UE1={the second serving cell}. In contrast, if first tofourth serving cells are configured for the second UE, it may result inAG1_UE2={the first serving cell, the second serving cell} andTAG2_UE2={the third serving cell, the fourth serving cell}.

For another example, the BS may configure the TAG in a cell-specificmanner. Network deployment information is determined irrespective of aUE. Thus, if the network deployment information is used as classifyingassistant information, a TAG may be configured in a cell-specific mannerirrespective of a UE. For example, it is assumed that the first servingcell of a specific frequency band is always served by a frequencyselective repeater or a remote radio head and the second serving cell ofa specific frequency band is served through a BS. In this case, thefirst serving cell and the second serving cell are classified asdifferent TAGs for all UEs within the service area of the BS.

The BS transmits TAG configuration information to the UE at step S510.The TAG configuration information includes information describing thestate in which TAGs are configured, and includes classifying at leastone serving cell configured in the UE into a TAG.

For example, the TAG configuration information may include the numberfield of the TAG, the index field of the TAG, and the index field ofeach of the serving cells included in the TAG, and the fields describethe state in which the TAG has been configured.

For another example, the TAG configuration information may furtherinclude information on a representative serving cell within each TAG. Arepresentative serving cell is a serving cell capable of performing arandom access procedure for maintaining and configuring uplinksynchronization within each TAG. The representative serving cell mayalso be called a special serving cell (special SCell) or a referenceserving cell (reference SCell). Unlike in the above embodiment, if TAGconfiguration information does not include a representative servingcell, a UE may select a representative serving cell within each TAG.

The UE performs a random access procedure for the BS at step S515.

FIG. 6 is a flowchart illustrating a method of transmitting TAGconfiguration information according to another example of the presentinvention.

Referring to FIG. 6, if a UE in a Radio Resource Control (RRC) idle modecannot aggregate CCs and only a UE in an RRC connected mode canaggregates CCS, a relevant UE selects a cell for RRC connection prior tothe CC aggregation and performs an RRC connection establishmentprocedure for a BD through the selected cell at step S600. The RRCconnection establishment procedure is performed in such a manner thatthe UE transmits an RRC connection request message to the BS, the BStransmits RRC connection setup to the UE, and the UE transmits an RRCconnection establishment completion message to the BS. The RRCconnection establishment procedure includes the configuration of anSRB1.

Meanwhile, the cell for RRC connection is selected using the followingselection conditions as criteria.

(i) The most suitable cell which will attempt RRC connection may beselected on the basis of information measured by a UE. As themeasurement information, the UE takes both RSRP regarding measuredreceive power on the basis of the received Cell-specific ReferenceSignal (CRS) of a specific cell and RSRQ defined as a ratio of theentire receive power (numerator) to an RSRP value (denominator) for thespecific cell into consideration. Accordingly, the UE obtains the RSRPand RSRQ values for each of distinguishable cells and selects the mostsuitable cell based on the RSRP and RSRQ values. For example, the UE mayselect a cell whose RSRP and RSRQ values are 0 dB or more and RSRP valueis a maximum, the UE may select a cell whose RSRQ value is a maximum, orthe UE may set (e.g., 7:3) weight to each of the RSRP and RSRQ valuesand select a cell on the basis of a mean value by taking the weight intoconsideration.

(ii) A UE may attempt RRC connection by using information which isstored in its internal memory and related to a service provider (PLMN)fixedly set in a system or downlink center frequency information or cellID information (e.g., a Physical cell ID (PID)). The stored informationmay include pieces of information about a plurality of service providersand cells, and priority or priority weight may be set to each of thepieces of information.

(iii) A UE may receive system information transmitted by a BS through abroadcasting channel, check information within the received systeminformation, and attempt RRC connection based on the checkedinformation. For example, a UE has to check whether a cell is a specificcell (e.g., a Closed Subscribe Group (CSG) or a non-allowed Home BS)requiring a membership for cell access. Accordingly, the UE receivessystem information transmitted by each BS and checks CSG ID informationindicating a CSG. If the specific cell is checked to belong to the CSG,the UE checks whether the CSG is an accessible CSG. In order to checkthe accessibility, the UE may use its own membership information andunique information about a CSG cell (e.g., (Evolved) Cell Global ID((E)CGI or PCI information within system information). If the specificcell is determined as an inaccessible BS through the check procedure,the UE does not attempt RRC connection.

(iv) A UE may attempt RRC connection through valid CCs stored in itsinternal memory (e.g., CCs that may be configured within a frequencyband supportable by the UE in implementation).

The conditions (ii) and (iv) of the four selection conditions areoptional, but the conditions (i) and (iii) condition are mandatory.

In order to attempt RRC connection through a cell selected for the RRCconnection, a UE has to check an uplink band on which an RRC connectionrequest message will be transmitted. Accordingly, the UE receives systeminformation through a broadcasting channel transmitted through thedownlink of the selected cell. A System Information Block 2 (SIB2)includes bandwidth information and center frequency information about aband to be used as UL. Accordingly, the UE attempts RRC connectionthrough the downlink of the selected cell and the uplink band linked tothe downlink through pieces of information within the SIB2. Here, the UEmay transfer the RRC connection request message to the BS as uplink datawithin a random access procedure. If the RRC connection procedure issuccessful, an RRC connection-established cell may be called a PSC, andthe PSC includes a DL PCC and a UL PCC.

If more radio resources have to be allocated to the UE at the request ofthe UE, at the request of a network, or according to the determinationof the BS, the BS performs an RRC connection reconfiguration procedurefor additionally configuring one or more SSCs (SCells) in the UE at stepS605. The RRC connection reconfiguration procedure is performed in sucha manner that the BS transmits an RRC connection reconfiguration messageto the UE and the UE transmits an RRC connection reconfigurationcompletion message to the BS.

The steps S500, S505, S510, and S515 are likewise applied to thefollowing steps S610, S615, S620, and S625, respectively. Meanwhile, theclassifying assistant information may be included in the RRC connectionreconfiguration completion message at step S605. In this case, the stepS610 may be omitted. Furthermore, the step S625 of performing a randomaccess procedure may be performed in a non-contention-based orcontention-based manner.

FIG. 7 is a flowchart illustrating a method of transmitting TAGconfiguration information according to yet another example of thepresent invention.

Referring to FIG. 7, a UE and a BS perform an RRC connectionestablishment procedure for the BS through a selected cell at step S700.The UE transmits classifying assistant information to the BS at stepS705. The classifying assistant information provides information or acriterion necessary to classify one or more serving cells, configured inthe UE, as a TAG. Meanwhile, the BS may have known the classifyingassistant information separately or may already have owned theclassifying assistant information. In this case, the random accessprocedure according to the present embodiment may be performed withoutthe step S705.

The BS configures a TAG by classifying the serving cells at step S710.The serving cells may be classified and configured into TAGs based onthe classifying assistant information.

If more radio resources have to be configured in the UE at the requestof the UE, at the request of a network, or according to thedetermination of the BS, the BS performs an RRC connectionreconfiguration procedure for additionally configuring one or more SSCsin the UE at step S715. In the RRC connection reconfiguration procedure,the BS may transmit an RRC connection reconfiguration message, includingTAG configuration information, to the UE. The TAG configurationinformation describes the state in which the TAG has been configured.For example, the TAG configuration information may include the numberfield of the TAG, the index field of the TAG, and the index field ofeach of the serving cells included in the TAG, and the fields describethe state in which the TAG has been configured.

Next, the UE performs a random access procedure at step S720. The randomaccess procedure may be performed in a non-contention-based orcontention-based manner.

The UE checks a TAC and/or a TAG index within the random access responsemessage and adjusts uplink timing regarding all the serving cells withinthe checked TAG by a TAV according to the TAC. An example of the uplinktiming adjusted by the TAV is shown in Equation 1 to Equation 4. If TACsand/or TAG indices for a plurality of TAGs exist in the random accessresponse message, the UE adjusts uplink timing regarding serving cell(s)for each TAG by a TAV according to the relevant TAC.

FIG. 8 is a flowchart illustrating a method of transmitting TAGconfiguration information according to further yet another example ofthe present invention.

Referring to FIG. 8, a UE and a BS perform an RRC connectionestablishment procedure for the BS through a selected cell at step S800.The selected cell becomes a PSC. If more radio resources have to beconfigured in the UE at the request of the UE, at the request of anetwork, or according to the determination of the BS, the BS performs anRRC connection reconfiguration procedure for additionally configuringone or more SSCs at step S805.

The UE configures one or more SSCs and performs a random accessprocedure at step S810. The UE transmits a random access preamble to theBS in order to secure timing synchronization for an SSC whosesynchronization has not been secured or a newly added or changed SSC.The random access procedure for the SSC may be initiated by a PDCCHorder transmitted by the BS. The random access procedure may beperformed in a non-contention-based manner or in a contention-basedmanner according to an intention of the BS.

The BS classifies serving cells configured in the UE on the basis of therandom access preamble received at step S810 and configures a TAG basedon the classified serving cells at step S815. The TAG is a groupincluding at least one serving cell, and the same TAV is applied toserving cells within a TAG. For example, the BS may configure the TAG ina UE-specific manner. For another example, the BS may configure the TAGin a cell-specific manner.

The BS transmits TAG configuration information to the UE at step S820.The TAG configuration information describes the state in which the TAGhas been configured.

For example, the TAG configuration information may include the numberfield of the TAG, the index field of the TAG, and the index field ofeach of the serving cells included in the TAG, and the fields describethe state in which the TAG has been configured.

For another example, the TAG configuration information may furtherinclude information on a representative serving cell within each TAG.The representative serving cell is a serving cell which may perform arandom access procedure for maintaining and setting uplinksynchronization within each TAG. Unlike in the above embodiment, if theTAG configuration information does not include a representative servingcell, the UE may independently select a representative serving cellwithin each TAG.

TAG configuration information is described below. For example, a BS maytransmit TAG configuration information to a UE using an RRC message. Forexample, the TAG configuration information may be included in an RRCconnection reconfiguration message used in an RRC connectionreconfiguration procedure and then transmitted. Table 1 shows an exampleof the RRC connection reconfiguration message included in the TAGconfiguration information.

TABLE 1 TAG-ConfigDedicated ::= SEQUENCE {   pTAG   SCellListOfTAG,  sTAG   SCellListOfTAG,   sTAG-referenceCell INTEGER (1..7) }SCellListOfTAG ::= SEQUENCE (SIZE (1..7)) OF Serv-index

Referring to Table 1, the RRC connection reconfiguration messageincludes TAG configuration information ‘TAG-ConfigDedicated’. Arepresentative serving cell index ‘referenceCell’ in the sTAG has anyone of values 1 to 7 and corresponds to a serving cell index. Theserving cell list information ‘SCellListOfTAG’ of the TAG has any one ofvalues 1 to 7 and corresponds to a serving cell index.

Table 2 shows another example of the RRC connection reconfigurationmessage including the TAG configuration information.

TABLE 2 TAG-ConfigDedicated ::= SEQUENCE {   pTAG   BIT STRING (SIZE(7)),   sTAG   BIT STRING (SIZE (7)),   sTAG-referenceCell INTEGER(1..7) }

Referring to Table 2, the RRC connection reconfiguration messageincludes TAG configuration information ‘TAG-ConfigDedicated’. Unlike inTable 1, in Table 2, serving cells included in a pTAG and an sTAG arerepresented by bit strings. The bit string has 7 bits, and one servingcell corresponds to only one bit of the bit string. However, the size ofthe bit string is not limited to 7 bits, and the bit string may havebits less or more than 7 bits.

For another example, a BS may transmit TAG configuration information toa UE using an MAC message.

The MAC message including the TAG configuration information may have anMAC PDU format shown in FIG. 17. In particular, the TAG configurationinformation may be included in an MAC CE, and the value of an LCID fieldindicating the MAC CE may be defined as in Table 3.

TABLE 3 LCID Index LCID Value 00000 CCCH 00001-01010 Identifier of alogical channel 01011-11001 Reserved 11010 TAG configuration information11011 Activation/deactivation 11100 UE contention resolution identifier11101 TAC 11110 DRX order 11111 Padding

Referring to Table 3, the LCID field having a value ‘11010’ indicatesthat a relevant MAC CE is an MAC CE including TAG configurationinformation (i.e., an MAC CE for a TAG). A MAC sub-header correspondingto the MAC CE for a TAG includes 6 fields; R/R/E/LCID/F/L. Here, the MACCE for a TAG may have a variable length. The L field indicates thelength of the MAC CE for a TAG in bytes. Furthermore, the length of theL field is indicated by the F field. For example, if the F field is 1bit and ‘1’, it means that the MAC CE for a TAG is smaller than 128bytes. In this case, an MAC sub-header is the same as that described inEmbodiment 1 of FIG. 9. Furthermore, if the F field is ‘0’, it indicatesthat the length of the MAC CE for a TAG is 128 bytes or more. In thiscase, an MAC sub-header is the same as that described in Embodiment 2 ofFIG. 9.

FIG. 10 is a diagram showing an MAC CE for a TAG according to an exampleof the present invention.

Referring to FIG. 10, an octet 1 (Oct 1) having 8 bits is a regioncorresponding to a pTAG, and it represents a serving cell included inthe pTAG in a bitmap form or a binary form. R, C7, C6, C5, C4, C3, C2,and C1 within the octet 1 sequentially correspond to a serving cellindex1, a serving cell index2, . . . , a serving cell index7 from theright, and R is a reserved field. That is, C_(n) corresponds to aserving cell index n. For example, if C_(n)=1, it may indicate that aserving cell having an index n is included in a pTAG. If C_(n)=0, it mayindicate that a serving cell having an index n is not included in apTAG. In the pTAG, a PSC always becomes a representative serving cell.

The octet 2 (Oct 2) is a region corresponding to a first sTAG, and itrepresents a serving cell, including in a sTAG, in a bitmap form or abinary form. R, C7, C6, C5, C4, C3, C2, and C1 within the octet 2sequentially correspond to a serving cell index1, a serving cell index2,. . . , a serving cell index7 from the right, and R is a reserved field.A next octet 3 (Oct 3) indicates a representative serving cell in thesTAG which is indicated by the octet 2, that is, an octet right beforethe octet 3. That is, the octet 3 includes a cell index field thatindicates the representative serving cell of the first sTAG. Since 7serving cells can be represented by 3 bits, the cell index field has 3bits, and the remaining five bits of the octet 3 are set for reservedfields R.

Likewise, an octet 2(N−1) is a region corresponding to an N^(th) sTAG,and an octet 2N−1 is a region indicating a representative serving cellin the N^(th) sTAG.

The R field has been illustrated as being placed in the left most bit ineach octet, but this is only illustrative. The R field may be placed inthe right most bit in each octet.

FIG. 11 is a diagram showing an MAC CE for a TAG according to anotherexample of the present invention.

Referring to FIG. 11, octets 1, 2, 3, . . . , n+1 are respective regionssequentially corresponding a pTAG, an sTAG1, an sTAG2, . . . , sTAGn andrepresent serving cells included in a TAG in a bitmap form or a binaryform. The MAC CE of FIG. 11 differs from that of FIG. 10 in that a cellindex field indicating the representative serving cell of each TAG doesnot exist. The representative serving cell of each TAG may be previouslydefined between a UE and a BS or may be informed by a UE throughadditional signaling. The R field has been illustrated as being placedin the left most bit in each octet, but this is only illustrative. The Rfield may be placed in the right most bit in each octet.

In FIG. 5 to FIG. 8, the uplink synchronization is not always performedin association with TAG configuration procedure, can be performedindependently of the TAG configuration procedure. In addition, a randomaccess procedure is disclosed for uplink timing synchronization as anexemplary purpose. The transmission and reception of a MAC message canalternatively be used for the uplink synchronization regardless of therandom access procedure.

As an example of the present invention, the method of transmitting andreceiving TAG Index (or TAG ID) and TAC is disclosed hereinafter inassociation with the random access procedure. The random accessprocedure may be performed in a non-contention-based or contention-basedmanner. The random access procedure has a different procedure accordingto whether it is performed in a non-contention-based or contention-basedmanner, and thus the drawings are changed as follows. A procedure ofFIG. 12 is applied in case of non-contention and a procedure of FIG. 16in case of contention.

FIG. 12 is a flowchart illustrating for reception of Timing AlignmentCommand (TAC) using a random access procedure according to an example ofthe present invention, which shows a non-contention-based random accessprocedure.

Referring to FIG. 12, a BS selects one of reserved and dedicated randomaccess preambles for a non-contention-based random access procedure,from among all available random access preambles, and transmits preambleallocation information (i.e., RA preamble assignment), including theindex of the selected random access preamble and availabletime/frequency resources information, to a UE at step S1200. The UEneeds to receive a dedicated random access preamble without a collisionpossibility from the BS for the non-contention-based random accessprocedure.

For example, if a random access procedure is performed during a handoverprocess, a UE may obtain a dedicated random access preamble from ahandover command message. For another example, if a random accessprocedure is performed at the request of a BS, a UE may obtain adedicated random access preamble through a PDCCH, that is, physicallayer signaling. In this case, the physical layer signaling is adownlink Control Information (DCI) format 1A, and it may include fields,such as those listed in Table 4.

TABLE 4 Carrier Indicator Field (CIF): CIF - 0 or 3 bits. Flag foridentifying formats 0/1A - 1 bit (indicating the format 0 in case of 0and the format 1A in case of 1) If format 1A CRC is scrambled by C-RNTIand the remaining fields are set as follows, the format 1A is used for arandom access procedure initiated by a PDCCH order. Below -Localized/distributed VRB allocation flag - 1 bit and set to 0 Resourceblock allocation - ┌log₂(N_(RB) ^(DL)(N_(RB) ^(DL) + 1)/2┐ bits. Allbits are set to 1 Preamble index - 6 bits PRACH mask index (maskindex) - 4 bits All the remaining bits of the format 1A for simplescheduling allocation of a PDSCH codeword are set to 0

Referring to Table 4, the preamble index indicates one preamble selectedfrom dedicated random access preambles previously reserved for anon-contention-based random access procedure. The PRACH mask index isavailable time/frequency resource information. The availabletime/frequency resource information indicates different resourcesaccording to a Frequency Division Duplex (FDD) system and a TimeDivision Duplex (TDD) system as in Table 5.

TABLE 5 PRACH Mask Index Permitted PRACH (FDD) Permitted PRACH (TDD) 0All All 1 PRACH resource index 0 PRACH resource index 0 2 PRACH resourceindex 1 PRACH resource index 1 3 PRACH resource index 2 PRACH resourceindex 2 4 PRACH resource index 3 PRACH resource index 3 5 PRACH resourceindex 4 PRACH resource index 4 6 PRACH resource index 5 PRACH resourceindex 5 7 PRACH resource index 6 Reserved 8 PRACH resource index 7Reserved 9 PRACH resource index 8 Reserved 10 PRACH resource index 9Reserved 11 All even-numbered PRACH All even-numbered PRACHopportunities within time opportunities within time domain, domain,First PRACH resource index First PRACH resource within subframe indexwithin subframe 12 All odd-numbered PRACH All odd-numbered PRACHopportunities within time opportunities within time domain, domain,First PRACH resource index First PRACH resource within subframe indexwithin subframe 13 Reserved First PRACH resource index within subframe14 Reserved Second PRACH resource index within subframe 15 ReservedThird PRACH resource index within subframe

The UE transmits the allocated dedicated random access preamble to theBS through a representative serving cell at step S1205. Therepresentative serving cell is a serving cell which has been selected totransmit the random access preamble in a TAG configured in the UE.

The representative serving cell may be selected for each TAG.Furthermore, the UE may transmit the random access preamble on arepresentative serving cell within any one TAG, from among a pluralityof TAGs, or may transmit the random access preamble on representativeserving cells within two or more respective TAGs. For example, it isassumed that TAGs configured in a UE include a TAG1 and a TAG2, TAG1={afirst serving cell, a second serving cell, a third serving cell}, andTAG2={a fourth serving cell, a fifth serving cell}. If therepresentative serving cell of the TAG1 is the second serving cell andthe representative serving cell of the TAG2 is the fifth serving cell,the UE transmits the allocated dedicated random access preamble to a BSthrough the second serving cell or the fifth serving cell.

The random access preamble may be performed after the representativeserving cell is activated. Furthermore, a random access procedure for anSSC may be initiated by a PDCCH order transmitted by a BS. In thepresent embodiment, a non-contention-based random access procedure isdescribed as an example, but the present embodiment may also be appliedto a contention-based random access procedure according to an intentionof a BS.

If only a TAV for a TAG including a representative serving cell isobtained, a UE may use the obtained TAV as the TAV of all other servingcells included in the TAG. This is because the same TAV is applied toserving cells belonging to the same TAG. As described above, theredundancy, complexity, and overhead of a random access procedure can bereduced by precluding an unnecessary random access procedure in aspecific serving cell.

The BS transmits a random access response message to the UE at stepS1210. For example, the random access response message may include aTAC. Here, the random access response message can be transmitted in formof MAC RAR as an example, and this is data element configured in a MACPDU for random access. In accordance with the present invention, the MACRAR can be transmitted by including TAC, in other word, TAV foradjusting uplink synchronization for a UE.

A TAC field indicates a change of relative uplink timing for the currentuplink timing, and it is a multiple of an integer of a sampling timeT_(s), for example, 16T_(s).

In addition, The BS may transmit the Timing Alignment Command (TAC)field to the UE without using the random access response message at stepS1210. The Timing Alignment Command (TAC) can be transmitted with indexinformation for a TAG to which a TAV is applied. The TAC indicates a TAVwhich equally or identically adjusts uplink timing of all serving cellsin a TAG. The TAV can be given as a specific index. For another example,a random access response message includes the index of a TAG having arepresentative serving cell and a TAC for the TAG.

The UE checks or identifies a TAC and/or a TAG index and adjusts theuplink timing regarding all the serving cells within the checked TAG bya TAV according to the TAC. An example of the uplink timing adjustedaccording to the TAV is shown in Equation 1 to Equation 4. If TACsand/or TAG indices for a plurality of TAGs exist within the randomaccess response message, the UE adjusts the uplink timing regarding aserving cell(s) for each TAG by a TAV according to the relevant TAC.Herein the UE may also determine a TAC and/or a TAG index within therandom access response message.

A BS may check that what UE has sent a random access preamble throughwhat serving cell based on a received random access preamble andtime/frequency resources. Accordingly, the BS transmits a random accessresponse message to a UE through a Physical Downlink Shared Channel(PDSCH) indicated by a PDCCH which has been scrambled into a Cell-RadioNetwork Temporary Identifier (C-RNTI) of a UE.

Here, the C-RNTI is an identifier which is allocated to each UE by a BSin order for the UE to check RRC connection set up with the BS and tocheck scheduling information transmitted by the BS. A BS must allocate adifferent C-RNTI value to each user within each BS. A temporary C-RNTIis allocated to a UE through a random access response during acontention-based random access procedure performed for RRC connection.When the relevant random access procedure is finally successful, the UErecognizes the temporary C-RNTI as a C-RNTI. A PDCCH indicating a PDSCHto which a random access response message has been mapped may betransmitted through a representative serving cell itself or a schedulingcell for a representative serving cell. Furthermore, if a random accessresponse message is transmitted as an MAC Control Element (CE) for aTAC, that is, if a random access response message is transmitted as aMAC RAR consisting of contents including a TAV (6 bits or 11 bits) andTAG ID information, a PDCCH order and the random access response messagemay be received through another serving cell not an SSC including UL inwhich the random access preamble has been transmitted. That is, therandom access response may be transmitted without being limited toscheduling for a specific serving cell.

Downlink Control Information (DCI) within a PDCCH on which physicallayer (L1) information, indicating that an indicator (i.e., a PDCCHorder) indicating the random access procedure and a random accessresponse message, that is, an MAC layer message have been allocated towhat radio resources, is transmitted may be transmitted through a lowerlayer control channel defined as an Extended PDCCH (EPDCCH). The EPDCCHconsists of a Resource Block (RB) pair. Here, the RB pair is defined asRBs for two slots, respectively, which form one subframe. If each RB isformed of a pair, the RB may be called a pair. Here, the RBs forming theRB pair may not be formed of slots having the same time. Furthermore,the RBs forming the RB pair may be formed of RBs existing in the samefrequency band or may be formed of RBs existing in different frequencybands. This is described with reference to FIGS. 13 to 15.

FIG. 13 shows an example in which DCI is mapped to an EPDCCH accordingto the present invention.

Referring to FIG. 13, a downlink subframe includes a control region 1300and a data region 1305. A PDCCH 1310 is mapped to the control region1300, and the downlink subframe has a length of two to four OFDM symbolsin the time domain. An Extended PDCCH (EPDCCH) 1315 and a PDSCH 1320 aremapped to the data region 1305. An indication relation between downlinkphysical channels is described below. The PDCCH 1310 indicates thetransmission of the EPDCCH 1315, and the EPDCCH 1315 indicates the PDSCH1320 including user information that is actually transmitted. The EPDCCH1315 and the PDCCH 1310 may be mapped to different DL CCs and may besubject to cross-carrier scheduling by the PDCCH 1310. However, theEPDCCH 1315 and the PDSCH 1320 exist in the same DL CC. The EPDCCH 1315may transmit a PDCCH order and DCI about physical layer (L1) informationof a random access response message.

FIG. 14 shows another example in which DCI is mapped to an ExtendedPDCCH according to the present invention.

Referring to FIG. 14, a PDCCH 1410 mapped to a control region 1400indicates an EPDCCH search space 1423 mapped to a data region 1405. A UEhas to detect the EPDCCH within the EPDCCH search space 1423 by using ablind decoding method used to receive the PDCCH 1410, that is, a datadetection method based on a Cyclic Redundancy Check (CRC) method.Furthermore, the EPDCCH 1423 and the PDCCH 1410 may be mapped todifferent DL CCs and may be subject to cross-carrier scheduling by thePDCCH 1410. The EPDCCH 1423 includes a PDCCH order and physical layer(L1) information about a random access response message.

FIG. 15 shows yet another example in which DCI is mapped to an extendedphysical downlink control channel according to the present invention.

Referring to FIG. 15, an EPDCCH 1505 exists in an EPDCCH search space1510 irrespective of a PDCCH. Information about the EPDCCH search space1510 provides information about a different search space (e.g., searchspace bandwidth information) to each UE through higher layer RRC orprovides information about a search space, shared by a plurality of UEs,through RRC signaling or a broadcasting method. Here, a control region1500 may do not exist, that is, may be removed.

In this case, a UE must perform blind decoding for the EPDCCH searchspace 1510 in order to obtain the EPDCCH 1505. If the EPDCCH searchspace 1510 is 1, that is, the EPDCCH search space 1510 has been definedas a space to which only one EPDCCH may be mapped, a method ofdetermining whether its own EPDCCH may be received according to a datadetection method using C-RNTI allocated to each UE may be used.Furthermore, the EPDCCH 1505 and a PDSCH 1515 exist in the same DL CC.

Whether a UE will receive the EPDCCH 1505 or a PDCCH from a relevantserving cell is determined by a BS. This may be configured for eachserving cell through higher layer RRC signaling.

If a UE has been configured to receive the EPDCCHs 1315, 1423, and 1505from a specific serving cell, the UE does not receive a PDCCH which istransmitted in a UE-specific manner. Accordingly, the UE may receive arandom access initiation indicator, including preamble allocationinformation, through only the EPDCCHs 1315, 1423, and 1505 in a randomaccess procedure which is performed in the specific serving cell.Furthermore, the UE may receive random access response informationwithin PDSCHs 1320, 1405, and 1515 which are indicated by the respectiveEPDCCHs 1315, 1423, and 1505. Therefore, the UE may also determine a TACand/or a TAG index in the random access response message within PDSCHs1320, 1405, and 1515 which are indicated by the respective EPDCCHs inthe present embodiments.

Referring back to FIG. 12, according to the non-contention-based randomaccess procedure, by receiving the random access response message, theUE determines that the random access procedure has been normallyperformed, and thus terminates the random access procedure. If apreamble index within preamble allocation information received by a UEis ‘000000’, the UE randomly selects one of contention-based randomaccess preambles, also sets a PRACH mask index value to ‘0’, and thenperforms a contention-based procedure. Furthermore, preamble allocationinformation may be transmitted to a UE through the message (e.g.,Mobility Control Information (MCI) within a handover command) of ahigher layer, such as RRC.

FIG. 16 is a flowchart illustrating for reception of Timing AlignmentCommand (TAC) using a random access procedure according to anotherexample of the present invention. This corresponds to a contention-basedrandom access procedure. A UE requires uplink synchronization in orderto transmit and receive data to and from a BS. The UE may perform aprocess of receiving information necessary for synchronization from theBS for the uplink synchronization. A random access procedure may also beapplied to the case where a UE is newly combined with a network forhandover, etc. After the UE is combined with the network, varioussituations, such as a change of the state of synchronization or RRC fromRRC_IDLE to RRC_CONNECTED, may be performed.

Referring to FIG. 16, a UE randomly selects one preamble signature froma set of random access preamble signatures and transmits a random accesspreamble according to the selected preamble signature to a BS through arepresentative serving cell by using PRACH resources at step S1600. Therepresentative serving cell is a serving cell selected to transmit therandom access preamble, from a TAG configured in the UE.

The meaning that the random access preamble has been transmitted maymean that pieces of configuration information for transmitting apreamble have been received in the random access procedure from allserving cells within a relevant TAG and may mean that a random accessprocedure has been instructed through a PDCCH order from among aplurality of serving cells within a TAG in which a random accessprocedure is possible. Furthermore, the representative serving cell maybe a serving cell including a timing reference DL CC which is acriterion in order to apply a TAV that is subsequently received througha random access response.

The representative serving cell may be selected for each TAG.Furthermore, the UE may transmit a random access preamble on arepresentative serving cell within any one TAG, from among a pluralityof TAGs, or may transmit a random access preamble on representativeserving cells within two or more TAGs, respectively.

The random access procedure may be performed after the representativeserving cell is activated. Furthermore, a random access procedure for anSSC may be initiated by a PDCCH order which is transmitted by the BS.

Information about a set of random access preambles may be obtained fromsome of system information or from a BS through a handover commandmessage. Here, the UE may know a Random Access-Radio Network TemporaryIdentifier (RA-RNTI) by taking frequency resources, temporarily selectedin order to select a preamble or transmit an RACH, and a transmissiontime into consideration.

In response to the random access preamble of the UE, the BS transmits arandom access response message to the UE at step S1605. A channel usedat this time is a PDSCH. The random access response message may betransmitted in the form of a MAC RAR. The random access response messageincludes a TAC for the uplink synchronization of the UE, UL radioresource allocation information, a Random Access Preamble IDentifier(RAPID) for identifying UEs each of which performs a random accessprocedure, information about a time slot on which the random accesspreamble of the UE has been received, and the temporary identifier ofthe UE, such as a temporary C-RNTI. The random access preambleidentifier is used to identify a received random access preamble.

Herein, The BS may transmit the Timing Alignment Command (TAC) field tothe UE without using the random access response message at step S1610.The Timing Alignment Command (TAC) can be transmitted with indexinformation for a TAG to which a TAV is applied. The TAC field indicatesthe TAV which equally or identically adjusts uplink timing of allserving cells in a TAG. The TAV can be given as a specific index. Foranother example, a random access response message includes the index ofa TAG having a representative serving cell and a TAC for the TAG.

The UE transmits uplink data, including the random access identifier, tothe BS through a PUSCH at the time of scheduling which is determinedbased on a TAV according to a TAC at step S1610. Here the UE maycheck/identify a TAC and/or a TAG index and adjust the uplink timingregarding all the serving cells within the checked TAG using the TAVaccording to the TAC field, before transmitting the uplink data. Anexample of the uplink timing adjusted according to the TAV is shown inEquation 1 to Equation 4. If TACs and/or TAG indices for a plurality ofTAGs exist within the random access response message, the UE adjusts theuplink timing regarding a serving cell(s) for each TAG by a TAVaccording to the relevant TAC field. Of course, the UE may alsodetermine a TAC and/or a TAG index within the random access responsemessage.

The uplink data may include an RRC connection request, tracking areaupdate, a scheduling request, or buffer status reporting on data thatwill be transmitted by the UE in UL. The random access identifier mayinclude a temporary C-RNTI, a C-RNTI (i.e., the state included in theUE), or UE identifier information (i.e., UE contention resolutionidentifier). When the TAV is applied, the UE starts or restarts a TAT.If the TAT is previously operated, the UE restarts the TAT. If the TATis not previously operated, the UE starts the TAT.

The BS transmits a contention resolution message, informing that therandom access has been successfully finished, to the UE because randomaccess preambles transmitted by several UEs may collide against eachother in the processes S1600 to S1610 at step S1615. The contentionresolution message may include a random access identifier. In acontention-based random access procedure, contention is generatedbecause the number of possible random access preambles is limited. Sinceunique random access preambles cannot be assigned to all UEs within acell, each UE randomly selects one random access preamble from a set ofrandom access preambles and transmits the selected random accesspreamble. Accordingly, two or more UEs may select and transmit the samerandom access preamble through the same PRACH resources.

At this time, the transmission of uplink data all fails, or a BSsuccessfully receives only uplink data from a specific UE according tothe position or transmit power of each of the UEs. If the BSsuccessfully receives uplink data, the BS transmits a contentionresolution message by using a random access identifier including theuplink data. A UE which has received its own random access identifiermay know that contention resolution is successful. In a contention-basedrandom access procedure, what a UE knows the failure or success ofcontention is called contention resolution.

When the contention resolution message is received, the UE checkswhether the contention resolution message is for the UE. If, as a resultof the check, the contention resolution message is for the UE, the UEtransmits ACK to the BS. If, as a result of the check, the contentionresolution message is for another UE, the UE does not transmit responsedata. Even when downlink allocation is missed or a message is notdecoded, the UE does not transmit response data. Furthermore, thecontention resolution message may include a C-RNTI, UE identifierinformation, etc.

As another example of the present invention, the method of transmittingand receiving TAG Index (or TAG ID) and TAC is disclosed hereinafter inassociation with a MAC layer signaling. The UE can obtain the TAV foruplink synchronization by means of MAC layer signaling regardless of therandom access procedure. In the case of MAC layer signaling, a MAC PDUmay include a header including a subheader having a LCID field forindicating that a specific MAC CE includes TAC field, and the specificMAC CE may include the TAC field and TAG index field.

FIG. 17 shows a Medium Access Control Protocol Data Unit (MAC PDU)format to which the present invention is applied.

Referring to FIG. 17, the MAC PDU 1700 includes an MAC header 1710, oneor more MAC CEs 1720 to 1725, one or more MAC Service Data Units (SDUs)1730-1 to 1730-m, and padding 1740.

The MAC header 1710 includes one or more sub-headers 1710-1, 1710-2 to1710-k. Each of the sub-headers 1710-1, 1710-2 to 1710-k may correspondto the MAC SDUs 1730-1 to 1730-m or one the MAC CEs 1720 to 1725 or thepadding 1740 in a one-to-one manner. The sequence of the sub-headers1710-1, 1710-2 to 1710-k is the same as the sequence of the MAC SDUs1730-1 to 1730-m, the MAC CEs 1720 to 1725 or the padding 1740 withinthe MAC PDU 1700.

Each of the sub-headers 1710-1, 1710-2 to 1710-k may include fourfields; R, R, E, and LCID or may include six fields; R, R, E, LogicalChannel ID (LCID), F, and L. The sub-headers each including the fourfields correspond to the MAC CEs 1720 to 1725 or the padding 1740, andthe sub-headers including the six fields correspond to the MAC SDU.

A LCID field identifies logical channels corresponding to the MAC SDUs1730-1 to 1730-m or identifies the type of the MAC CEs 1720 to 1725 orthe padding 1740. When each of the sub-headers 1710-1, 1710-2 to 1710-khas an octet structure, the LCID field may have 5 bits.

For example, the LCID field may be used to identify whether thecorresponding MAC CE is a MAC CE for indicating theactivation/deactivation of a serving cell, contention resolutionidentity MAC CE for solving contention between UEs, or a MAC CE for aTAC as in Table 6 according to an LCID value. The MAC CE for a TAC is aMAC CE used for timing alignment, and it includes a TAC field.

TABLE 6 LCID Index LCID Value 00000 CCCH 00001-01010 Identifier of alogical channel 01011-11010 Reserved 11011 Activation/deactivation 11100UE contention resolution identifier 11101 TAC 11110 DRX order 11111Padding

Referring to Table 6, if an LCID field has a value 11101, an MAC CEcorresponding to a sub-header including the LCID field is an MAC CE fora TAC.

Meanwhile, when a TAC is given for a plurality of serving cells becausethe plurality of serving cells is configured for a UE, an LCID field maybe given as in Table 7.

TABLE 7 LCID index LCID value 00000 CCCH 00001-01010 Identifier of alogical channel 01011-11001 Reserved 11010 Extended Timing AdvanceCommand (TAC) 11011 Activation/deactivation 11100 UE contentionresolution identifier 11101 TAC 11110 DRX order 11111 Padding

Referring to Table 7, if an LCID field has a value 11010, a MAC CEcorresponding to a sub-header including the LCID field may be an MAC CEfor a TAC for a plurality of serving cell.

The MAC CEs 1720 to 1725 are control messages generated by an MAC layer.The padding 1740 is a specific number of bits which are added to makethe size of an MAC PDU constant. The MAC CEs 1720 to 1725, the MAC SDUs1730-1 to 1730-m, and the padding 1740 are also collectively called anMAC payload.

For uplink timing adjustment, a BS transmits the index of a specific TAGand a TAV, applied to the specific TAG in common, to a UE. To this end,an MAC CE for a TAC may be used. Examples of the MAC CE for a TAC areshown in FIG. 18 to FIG. 20.

FIG. 18 is a block diagram showing a MAC CE for a Timing AlignmentCommand (TAC) according to an example of the present invention.

Referring to FIG. 18, the MAC CE for a TAC includes the TAG index fields(G₁ and G₀) and a TAC field. Here, the index of the TAG may also becalled a TAG index or an TAG identifier (ID). The TAC field indicates aTAV. When the MAC CE for a TAC has an octet structure, the TAG indexfield has 2 bits, and the TAC field has 6 bits. The TAG index is definedin TAG configuration information. For example, the TAG index field (G₁,G₀) is used 2 bits, so there are 4 TAGs and TAG indices are 0, 1, 2, and3, G₁, G₀ may be expressed as one of {00, 01, 10, 11}.

If a maximum number of TAGs are 2, for example, any one of 2 bit-TAGindex field such as G₁ may be set as a reserved bit R, or a TAG indexfield may be defined to have only 1 bit and the TAC field may be definedto have 7 bits.

The number of bits of the TAG index field and the number of bits of theTAC field are only illustrative, but are not necessarily limited to 2and 6, respectively. Furthermore, the TAG index including a PSC may befixed to ‘00’ or ‘0’.

FIG. 19 is a block diagram showing a MAC CE for a TAC according toanother example of the present invention.

Referring to FIG. 19, the MAC CE for a TAC includes an octet 1 (Oct 1)to an octet N (Oct N). Each octet includes the TAG index fields G₁ andG₀ and a TAC field.

For example, it is assumed that a UE requires a TAV for a TAG1 and a TAVfor a TAG2 for uplink synchronization. A BS includes an MAC CE for a TACfor two octets in an MAC PDU, such as that shown in FIG. 17 (i.e., N=2).Here, the first octet includes the index field of the first TAG,indicating the index of the TAG1, and a first TAC field indicating theTAV of the TAG1. The second octet includes the index field of the secondTAG, indicating the index of the TAG2, and a second TAC field indicatingthe TAV of the TAG2.

In other words, the TAG index fields with 2 bits (G₁, G₀) in each Octetindicates a group to perform uplink timing adjustment, and the TAC fieldindicates a TAV for the corresponding group.

A UE can perform the same or adaptive timing alignment according to TAVconfigured for a plurality of groups by using a single message, that is,a MAC message which includes a multiple group indices and a multipleTAVs for the corresponding multiple groups. When a plurality of TAGs areconfigured in a UE, a MAC CE indicating TAVs for a plurality of TAGs canbe configured.

The UE performs timing alignment by applying the TAV of the TAG1 to allserving cells belonging to the TAG1 and performs timing alignment byapplying the TAV of the TAG2 to all serving cells belonging to the TAG2.

FIG. 20 is a block diagram showing a MAC CE for a TAC according to yetanother example of the present invention.

Referring to FIG. 20, the MAC CE for a TAC includes an octet 1 (Oct 1)to an octet N (Oct N). The octet 1 includes an indicator for eachserving cell (or a CC). The indicator may indicate whether a TAC fieldregarding a serving cell exists or not. For example, G0 becomes anindicator indicating whether a TAC field for a TAG0, that is, a TAGincluding a PSC exists or not, G1 becomes an indicator indicatingwhether a TAC field for a TAG1 exists or not, and G7 becomes anindicator indicating whether a TAC field for a TAG7 exists or not.Accordingly, the G0, G1, and G3 indicating the TAG0, TAG1, and TAG3 areset to 1, and the remaining G2 and G4˜G7 are set to 0. Furthermore, TACvalues for the TAG0, TAG1, and TAG3 are sequentially mapped. If thestructure of FIG. 20 is used, a TAC value may be set for a specific TAGrequiring update for the specific TAG. Each of the remaining octet 2 tooctet N includes reserved fields R and a TAC field.

FIGS. 21 and 22 are block diagrams showing MAC RARs for a TAC accordingto yet another example of the present invention.

Referring to FIG. 21 and FIG. 22, the MAC CE for a TAC includes indicesG1 and G0 of TAG fields and a TAC field. If the MAC CE for a TAC has atwo-octet structure, reserved bits may be 3 bits, the index of the TAGfield may have 2 bits, and the TAC field may have 11 bits. If the indexof the TAG field has 1 bit or 3 bits, reserved bits may have 4 bits or 2bits. Here, as in FIG. 22, one of the reserved bits may be set to abit(X) not using the most significant bit.

The TAC field with 11 bits is a message format used in a random accessprocedure, and thus transmitted by being included in a random accessresponse message.

FIG. 23 is a flowchart illustrating the operation of a UE which performsuplink synchronization according to an example of the present invention.

Referring to FIG. 23, the UE transmits assistant information to a BS atstep S2300. The assistant information provides information or acriterion necessary to classify one or more serving cells, configured inthe UE, into a TAG. Meanwhile, the BS may have known the classifyingassistant information separately or may already have owned theclassifying assistant information. In this case, the random accessprocedure according to the present embodiment may be performed withoutthe step S2300.

If a UE in an idle mode cannot aggregate CCs and only a UE in an RRCconnected mode can aggregate CCs, the UE in the idle mode may select acell for RRC connection prior to the aggregation of CCs before the stepS2300 and perform an RRC connection establishment procedure for the BSthrough the selected cell.

The UE receives TAG configuration information from the BS at step S2305.The TAG is a group including at least one serving cell, and the same TAVis applied to serving cells within a TAG. In other words, a single TAVis identically applied to all serving cells in the TAG. For example, theBS may configure the TAT in a UE-specific manner. For another example,the BS may configure the TAG in a cell-specific manner. The TAGconfiguration information can be received via RRC signaling. Or The TAGconfiguration information can be received via MAC signaling.

The TAG configuration information describes the state in which the TAGhas been configured. For example, the TAG configuration information mayinclude the number field of the TAG, the index field of the TAG, and theindex field of each of the serving cells included in the TAG, and thefields describe the state in which the TAG has been configured.

For another example, the TAG configuration information may furtherinclude information on a representative serving cell within each TAG.The representative serving cell is a serving cell which may perform arandom access procedure for maintaining and setting uplinksynchronization within each TAG. Unlike in the above embodiment, if theTAG configuration information does not include a representative servingcell, the UE may independently select a representative serving cellwithin each TAG.

Next, the UE performs a random access procedure at step S2310. In thisstep, the UE transmits a random access preamble to the BS and mayreceive a random access response message, including a TAC field, fromthe BS. For example, the UE may receive the random access responsemessage in form of a MAC RAR in response to the random access preamble.The MAC RAR includes the TAC field, which indicates a TAV. The UE canalso receive index information of a TAG (that is, TAG ID) to performuplink timing alignment.

Another example, the UE may receive a TAC field from the BS at stepS2315 after receiving the random access response message (at stepS2310). Also the UE can receive a TAC field from the BS at step S2315without receiving the random access response message at step S2310.

In more detail, the UE can receive the TAC field via a MAC PDU asdescribed in FIG. 18 to FIG. 20. The MAC PDU includes a header includinga subheader which corresponds to a MAC CE, the MAC CE having a TAG indexfield and the TAC field. Accordingly, the UE can obtain the subheaderand the MAC CE including the TAG index for the TAC field from the MACPDU. That is, the UE obtains the TAG index field and the TAC field fromthe MAC CE after checking that the LCID index is set to 11101 as LCIDvalue for TAC. The MAC CE may also include TAVs each of whichcorresponds to each of TAGs respectively.

According to the present invention, the UE checks an index of a LCIDfield, which is included in the subheader and corresponds to the MAC CE,and the UE identifies that the MAC CE is for a TAC based on thechecking. That is, the UE checks whether the LCID field indicates aspecific index (i.e. 11101) which represents that the corresponding MACCE includes the TAC field.

And as a result of the checking, if the MAC CE is for a TAC, the UEdetermines (or acquires or identify) the TAV indicated by the TAC fieldin the MAC CE. The UE identifies a TAG by the TAG index field. The UEcan identify 4 TAGs defined by 2 bits of the TAG index fields (G₁, G₀).The UE adjusts uplink timing of serving cell(s) in the TAG identicallywith a TAV indicated by the TAC field. In other words, the UE controlsuplink timing regarding at least one serving cell or all serving cellswithin the identified TAG by the TAV. Accordingly, the UE performs anuplink timing adjustment operation. In an example, if the UE finds thatthe (G₁, G₀) is set to 00, the UE controls uplink timing of servingcells in a pTAG, which includes PSC, by applying the TAV.

If the MAC CE for a TAC includes TAC fields and/or TAG index fields formultiple TAGs as described in FIG. 19 or FIG. 20, the UE adjusts uplinktiming of serving cell(s) in each TAG based on a TAV indicated by eachof the TAC fields. An example of the uplink timing adjusted by the TAVis shown in Equation 1 to Equation 4.

FIG. 24 is a flowchart illustrating the operation of a BS which performsuplink synchronization according to an example of the present invention.

Referring to FIG. 24, the BS receives assistant information from a UE atstep S2400. The assistant information provides information or acriterion necessary to classify one or more serving cells, configured inthe UE, as a TAG. Meanwhile, the BS may have known the classifyingassistant information separately or may already have owned theclassifying assistant information. In this case, the random accessprocedure according to the present embodiment may be performed withoutthe step S2400.

The BS configures the TAG based on the classifying assistant informationat step S2405 and transmits TAG configuration information to the UE atstep S2410. The TAG is a group including one or more serving cells. Thesame TAV is applied to serving cell(s) within a TAG. For example, the BSmay configure the TAG in a UE-specific manner. For another example, theBS may configure the TAG in a cell-specific manner. The TAGconfiguration information describes the state in which the TAG has beenconfigured.

Next, the BS may perform a random access procedure at step S2415. Inthis step, the BS receives a random access preamble from the UE andtransmits a random access response message to the UE. The BS maytransmit the Timing Alignment Command (TAC) field to the UE withoutusing the random access response message at step S2415. Here, the randomaccess response message is transmitted by including a MAC RAR and acorresponding subheader, wherein the MAC RAR is a data component of therandom access response message, and the corresponding subheader includesa random access preamble identifier (RAPID) field. According to thepresent invention, the BS may include a TAC field in the MAC RAR whentransmitted.

Apart from the step S2415, the BS may transmit TAC field to the UE atstep S2420. In more detail, the BS may also determine a TAC and/or a TAGindex and transmit the TAC and/or a TAG index for the TAC. the BSgenerates a MAC PDU by including a TAC field as described in FIG. 18 toFIG. 20 in the MAC PDU. The BS transmits the MAC PDU by including aheader and a MAC CE, wherein the header includes a subheader whichcorresponds to a MAC CE and the MAC CE includes a TAG index field andthe TAC field. Herein, The BS may set the subheader of LCID value forthe TAC to send the MAC PDU for the TAC without the random accessresponse message.

The BS sets the TAG index fields (G₁, G₀) to indicate 4 TAGs defined by2 bits. That is, the maximum number of TAGs configurable in the UE is 4by the TAG index fields (G₁, G₀) of the length of 2 bits. The BSdetermines the TAG index field to indicate a certain TAG among the 4TAGs. And if the BS determines that uplink timing control is requiredfor a pTAG, the BS sets the (G₁, G₀) to 00. And the BS sets the TACfield to a TAV relevant to all serving cell(s) in the pTAG.

In other words, according to the present invention, the BS generates aMAC CE by including the TAC field and the TAG field in the MAC CE, andgenerates a header having a subheader with the LCID field, by setting(or configuring) the LCID value to a specific index (i.e. 11101) toindicate that the MAC CE corresponding to the subheader is for a TAC.The BS transmits the MAC PDU including the header and the MAC CE andtransmits the generated MAC PDU to the UE.

FIG. 25 is an explanatory diagram illustrating a method of performinguplink synchronization by using a TAV in a multiple CC system.

Referring to FIG. 25, reference time means time which is a criterion forthe synchronization of downlink or uplink. A serving cell which providesthe reference time may be called a timing reference cell. It is assumedthat the reference time has been set to a point of time at which adownlink frame is received by a UE, synchronized, and then checked. Aserving cell1 (SCell 1), a serving cell2 (SCell 2), a serving cell3(SCell 3), a serving cell4 (SCell 4), and a serving cell5 (SCell 5) areconfigured in the UE. For example, SCell1 can be configured as a PSC.

A BS configures the serving cell1, the serving cell3, and the servingcell4 as one TAG1 and configures the serving cell2 and the serving cell5as another TAG2. So the BS can configure the TAG1 including the PSC as apTAG. The current uplink timing of serving cells of the pTAG is laterthan the reference time by TA1. Thus, the BS sets a first TAV so thatthe uplink timing of the serving cells of the pTAG advances the presenttime by the TA1, indicates the first TAV in a first TAC field. The BSsets a first TAG index field to 00 to indicate the pTAG, and transmitsthe first TAC field and the first TAG index field to the UE.

Furthermore, the uplink timing of serving cells of the TAG2 (that is asTAG) is later than the present time by TA2. Thus, the BS sets a secondTAV so that the uplink timing of the serving cells of the TAG2 advancesthe present time by the TA2, indicates the second TAV in a second TACfield. The BS sets a second TAG index field to 01 to indicate the sTAG,and transmits the second TAC field and the second TAG index field to theUE.

A MAC CE including the first TAC field and the first TAG index field ora MAC CE including the second TAC field and the second TAG index fieldmay have any one of the structures shown in FIGS. 18 to 22.

The UE may calculate the TA1 and the TA2 by using the first and thesecond TAVs provided by the BS and adjust the uplink timing based on thecalculated TA1 and TA2. The adjusted uplink timing alignment (TA) may becalculated according to Equation 1 below.

TA=(N _(TA) +N _(TA offset))×T _(s)  [Equation 1]

In Equation 1, N_(TA) is a timing offset between a UL radio frame and adownlink radio frame in a UE and is indicated by a T_(s) unit. N_(TA) isvariably controlled by the TAC of the BS, and N_(TA) offset is a valuefixed by a frame format. T_(s) is a sampling period.

Meanwhile, an old timing offset N_(TA-old) is adjusted to a new timingoffset N_(TA-new) by a TAV T_(i) and N_(TA-new) may be calculatedaccording to Equation 2 below.

N _(TA-new) =N _(TA-old)(T _(i)−31)×16  [Equation 2]

Referring to Equation 2, T_(i) is an index value and is 0, 1, 2, . . . ,63. That is, T_(i) may be represented by 6 bits. T_(i) is indicated by aTAC field. Here, if N_(TA) is positive (+), it means that uplink timingis advanced. If N_(TA) is negative (−), it means that uplink timing isdelayed. That is, the TAC field indicates a TAV which is a relativechange of uplink timing to old uplink timing.

Alternatively, the TAV may also be used to determine the timing offsetN_(TA) of a TAG including an SSC to a change of the uplink timing of aTAG including a PSC, as in Equation 3.

N _(TA-TAG(Sn)) =N _(TA-TAG(p))+(T _(i-n)−31)×16  [Equation 3]

Referring to Equation 3, N_(TA-TAG(Sn)) is a timing offset for a TAGwhich does not include a PSC (PCell) and has an index value of n.N_(TA-TAG(p)) is a timing offset for a TAG including a PSC (PCell).T_(i-n) is a TAV T_(i) for a TAG having an index value of n.

If a UE receives a TAV for a serving cell for the first time, thecalculation of the timing offset NTA may be defined as in Equation 4because there is no value to be compared with the TAV.

N _(TA-TAG(Sn))=(T _(i-n)−31)×16  [Equation 4]

For another example, if the propagation delay time of downlinktransmission is identical with the propagation delay time of uplinktransmission, a UE may adjust uplink timing for all serving cells byusing the propagation delay time of the downlink transmission.

FIG. 26 is a block diagram showing a BS and a UE which perform uplinksynchronization according to an example of the present invention.

Referring to FIG. 26, the UE 2600 includes a UE reception unit 2605, aUE processor 2610, and a UE transmission unit 2620. The UE processor2610 includes an RRC processing unit 2611 and an MAC processing unit2612.

The UE reception unit 2605 receives an MAC PDU, preamble allocationinformation regarding a PDCCH order, TAG configuration information, anda MAC CE for a TAC, an RRC connection message, an RRC connectionreconfiguration message, or a contention resolution message from the BS2650.

The RRC processing unit 2611 configures and generates assistantinformation, an RRC connection message, and an RRC connectionreconfiguration completion message. Here, the assistant informationprovides information or a criterion on classifying at least one servingcell configured in the UE 2600 into a TAG. The assistant information maybe included in an RRC connection reconfiguration completion message oranother message. A representative serving cell is a serving cellincluding a timing reference DL CC, that is, a criterion in order toapply a TAV subsequently received through a random access response. TheTAG configuration information describes the state in which TAGs areconfigured in the UE 2600.

The RRC processing unit 2611 analyzes the TAG configuration informationreceived from the BS 2650, and identifies at least one of the number ofTAGs configured in the UE 2600, an index of each TAG, indices of servingcells included in each TAG and representative serving cell information.Or, the TAG configuration information may be analyzed by the MACprocessing unit 2612 if the TAG configuration information is transmittedas shown in FIGS. 9 to 11.

The MAC processing unit 2612 performs operations for uplink timingalignment. To acquire uplink timing synchronization, the MAC processingunit 2612 may receive or identify a MAC RAR according to a random accessprocedure or may receive or identify a MAC layer signaling. Here, theMAC layer signaling may include a MAC PDU including a MAC CE for a TAC.

In an embodiment, the MAC processing unit 2612 may process anon-contention-based or contention-based random access procedure. TheMAC processing unit 2612 generates a random access preamble to acquireuplink timing synchronization for a serving cell. The generated randomaccess preamble may be a dedicated random access preamble assigned bythe BS 2650. If multiple TAGs are configured in the UE 2600, the MACprocessing unit 2612 may generate multiple random access preambles to betransmitted on the representative serving cell of each TAG.

The MAC processing unit 2612 may obtain a TAC from the random accessresponse message in a MAC RAR format, and may obtain TAG indexinformation for which uplink timing alignment is performed.

In another embodiment, the MAC processing unit 2612 identifies andanalyzes the MAC PDU received by the UE reception unit 2605. Forexample, the MAC processing unit 2612 obtains a header, a MAC CE and aMAC SDU, etc from the MAC PDU. The MAC processing unit 2612 obtains,from the MAC header, a subheader. The MAC processing unit 2612 obtains,from the subheader, a LCID field corresponding to the MAC CE. The MACprocessing unit 2612 obtains the TAC field with 6 bits and the TAG indexfield with 2 bits.

More details, the MAC processing unit 2612 identifies the type of theMAC CE by analyzing an index indicated by the LCID field. Based on theindex indicated by the LCID field, the MAC processing unit 2612identifies that the MAC CE is for a TAC. For instance, the MACprocessing unit 2612 checks if the index of the LCID field is set to11101. If the index of the LCID field is set to 11101, the MACprocessing unit 2612 confirms that the MAC CE includes a TAC field foruplink timing alignment and a TAG index field to which a TAV indicatedby the TAC field is applied. In other words, the MAC processing unit2612 checks (or determines) a corresponding MAC CE for the TAC, such asthat shown in FIGS. 18 to 20, in the MAC PDU, based on the index of theLCID field included in the subheader.

The MAC processing unit 2612 identifies a TAG based on the TAG indexfields (G₁, G₀) and identifies the TAV based on the TAC field. Here, theTAG includes at least one serving cell to identically control uplinktiming adjustment according to the TAV. And the maximum number of TAGsmay be limited to 4. Then, the MAC processing unit 2612 determines toperform uplink timing alignment for a pTAG or a sTAG through the TAGindex fields (G₁, G₀). For example, if the TAG index fields (G₁, G₀) areset to 00, then the MAC processing unit 2612 performs uplink timingalignment for the pTAG. And the MAC processing unit 2612 can identify 4TAGs defined by 2 bits of the TAG index fields (G₁, G₀). And the MACprocessing unit 2612 adjusts uplink timing with regards to indication ofa TAC field included in the MAC CE.

The MAC processing unit 2612 performs uplink timing alignment bychecking a TAC field and a TAG index field within the MAC CE, andadjusts uplink timing regarding at least one serving cell within thechecked TAG according to a TAV indicated by the TAC.

An example in which the MAC processing unit 2612 adjusts the uplinktiming based on the TAV is shown in any one of Equation 1 to Equation 4.If TAC fields and TAG indices for a plurality of TAGs exist in the MACPDU, the MAC processing unit 2612 adjusts uplink timing regardingserving cells for each TAG according to a TAV indicated by a relevantTAC.

The UE transmission unit 2620 transmits assistant information, an RRCconnection message, an RRC connection reconfiguration completionmessage, or a random access preamble to the BS 2650. For example, it isassumed that TAGs configured in the UE are a TAG1 and a TAG2, TAG1={afirst serving cell, a second serving cell, a third serving cell}, andTAG2={a fourth serving cell, a fifth serving cell}. If a representativeserving cell within the TAG1 is the second serving cell and arepresentative serving cell within the TAG2 is the fifth serving cell,the UE transmission unit 2620 transmits a first random access preambleon the second serving cell and transmits a second random access preambleon the fifth serving cell. Furthermore, the UE transmission unit 2620may transmit an uplink signal based on uplink timing adjusted by the MACprocessing unit 2612.

The BS 2650 includes a BS transmission unit 2655, a BS reception unit2660, and a BS processor 2670. The BS processor 2670 includes an RRCprocessing unit 2671 and an MAC processing unit 2672.

The BS transmission unit 2655 transmits preamble allocation information,TAG configuration information, a random access response message, a MACPDU including a MAC CE for a TAC, an RRC connection completion message,an RRC connection reconfiguration message, or a contention resolutionmessage to the UE 2600. The random access response message istransmitted through a PDSCH indicated by a PDCCH scrambled with aC-RNTI. Here, the C-RNTI is an identifier allocated to the UE 2600 bythe BS 2650 in order for the UE 2600 to check RRC connection set up withthe BS 2650 and scheduling information transmitted by the BS 2650. TheBS 2650 has to allocate a different C-RNTI value to each user within theBS 2650. A temporary C-RNTI is allocated through a random accessresponse during a contention-based random access procedure which isperformed by the UE 2600 for RRC connection. When the relevant randomaccess procedure is finally successful, the UE 2600 recognizes thetemporary C-RNTI as a C-RNTI. The PDCCH indicating the random accessresponse message may be transmitted through a representative servingcell itself or a scheduling cell for a representative serving cell.

The MAC CE for a TAC may be transmitted, including a TAC field with 6bits or 11 bits and TAG ID information. The PDCCH indicating the randomaccess response message and the random access response message may bereceived through another serving cell not an SSC which includes a UL CCon which a random access preamble has been transmitted. That is, whenthe random access response is transmitted, the PDCCH indicating therandom access response message and the random access response messagemay be transmitted without being limited to scheduling for a specificserving cell.

The BS reception unit 2660 receives assistant information, a randomaccess preamble, a message related to RRC connection, or a messagerelated to an RRC connection reconfiguration from the UE 2600.

The RRC processing unit 2671 configures and generates an RRC connectioncompletion message or an RRC connection reconfiguration message.Furthermore, the RRC processing unit 2671 generates a TAG and generatesTAG configuration information.

The TAG is a group including at least one serving cell configured in theUE 2600, and the same TAV is applied to serving cells within a TAG. Forexample, the RRC processing unit 2671 may configure the TAG specificallyto the UE 2670. For another example, the RRC processing unit 2671 mayconfigure the TAG in a cell-specific manner.

The MAC processing unit 2672 selects one of dedicated random accesspreambles which have been reserved for a non-contention-based randomaccess procedure, from among all available random access preambles andgenerates preamble allocation information including the index of theselected random access preamble and information about availabletime/frequency resources.

In an embodiment, the MAC processing unit 2672 generates a random accessresponse message or a contention resolution message. The MAC processingunit 2672 checks a representative serving cell on which a random accesspreamble has been transmitted and checks a TAG including therepresentative serving cell. And the MAC processing unit 2672 maygenerate TAG configuration information as described in any one of FIGS.9 to 11.

In another embodiment, the MAC processing unit 2672 determines a TAVthat is to be applied to (or relevant to) the TAG and generates a TACfield with 6 bits indicating the determined TAV. The TAC indicates arelative change of uplink timing for the present uplink timing, and itmay be a multiple of an integer of a sampling time T_(s), for example,16 T_(s). The TAC may be represented by a TAV having a specific index.The MAC processing unit 2672 generates the MAC CE for a TAC includingthe generated TAC field and the corresponding TAG index field.

In more detail, the MAC processing unit 2672 generates (sets orconfigures) a LCID field with a specific index such as 11101 to indicatethe corresponding MAC CE is for a TAC, and generates a header having asubheader by including the LCID field in the subheader. The MACprocessing unit 2672 also generates the MAC CE for a TAC by including aTAC field and a TAG index field (G₁, G₀) in the MAC CE. The MACprocessing unit 2672 can identify 4 TAGs defined by 2 bits of the TAGindex fields (G₁, G₀). Furthermore, the MAC processing unit 2672generates the TAG fields (G₁, G₀) to ‘00’ or ‘0’ to indicate a PSC maybe included.

And the MAC processing unit 2672 generates a MAC PDU by including thegenerated MAC CE and the generated header in the MAC PDU. The MACprocessing unit 2672 generates and sets the TAG index field (G₁, G₀) toa specific value to identify that the TAG is a pTAG or a sTAG. Thestructure of the MAC PDU is shown in FIG. 17, and the structure of theMAC CE for the TAC may be any one of FIGS. 18 to 20.

In accordance with this specification, a procedure of obtaining a TAVfor a serving cell in order to secure and maintain uplink timingsynchronization becomes clear, the time taken to obtain uplinksynchronization for a serving cell may be reduced, and overhead may bereduced by obtaining a TAV for a plurality of serving cells.

While some exemplary embodiments of the present invention have beendescribed with reference to the accompanying drawings, those skilled inthe art may change and modify the present invention in various wayswithout departing from the essential characteristic of the presentinvention. Accordingly, the disclosed embodiments should not beconstrued to limit the technical spirit of the present invention, butshould be construed to illustrate the technical spirit of the presentinvention. The scope of the technical spirit of the present invention isnot limited by the embodiments, and the scope of the present inventionshould be interpreted based on the following appended claims.Accordingly, the present invention should be construed to cover allmodifications or variations induced from the meaning and scope of theappended claims and their equivalents.

1. A user equipment (UE) performing an uplink synchronization in awireless communication system, the UE comprising: a receiving unitconfigured for receiving a medium access control (MAC) protocol dataunit (PDU) from a base station (BS); and a MAC processing unitconfigured for: identifying that the MAC PDU includes a MAC header and aMAC control element (CE); obtaining, from the MAC header, a subheader;obtaining, from the subheader, a logical channel identifier (LCID)indicating an index to identify whether the MAC CE is for timing advancecommand (TAC); obtaining, from the MAC CE, a TAC field with 6 bitsindicating a timing alignment value (TAV) and a timing alignment group(TAG) index field with 2 bits identifying a TAG to control uplink timingadjustment identically with the TAV; identifying the TAG to which theTAV is applied based on the TAG index field; and controlling the uplinktiming adjustment for at least one serving cell in the identified TAG.2. The UE of claim 1, wherein the MAC processing unit is furtherconfigured for identifying the TAG among 4 TAGs defined by the 2 bits.3. The UE of claim 1, wherein the MAC processing unit is furtherconfigured for identifying that the TAG to include a primary servingcell (PSC) when the TAG index field is set to
 00. 4. The UE of claim 1,wherein the MAC processing unit is configured for identifying that theMAC CE includes the TAC field when the index indicated by the LCID isset to
 11101. 5. The UE of claim 1, further comprising: a radio resourcecontrol (RRC) processing unit configured for receiving, from the BS, aRRC message, the RRC message including a TAG configuration informationwhich includes at least one serving cell corresponding to at least oneTAG to be configured.
 6. A method of performing an uplinksynchronization by a user equipment (UE) in a wireless communicationsystem, the method comprising: receiving a medium access control (MAC)protocol data unit (PDU) from a base station (BS); identifying that theMAC PDU includes a MAC header and a MAC control element (CE); obtaining,from the MAC header, a subheader; obtaining, from the subheader, alogical channel identifier (LCID) indicating an index to identifywhether the MAC CE is for timing advance command (TAC); obtaining, fromthe MAC CE, a TAC field with 6 bits indicating a timing alignment value(TAV) and a timing alignment group (TAG) index field with 2 bitsidentifying a TAG to control uplink timing adjustment identically withthe TAV; identifying the TAG to which the TAV is applied based on theTAG index field; and controlling the uplink timing adjustment for atleast one serving cell in the identified TAG.
 7. The method of claim 6,wherein the TAG is identified among 4 TAGs defined by the 2 bits.
 8. Themethod of claim 6, wherein the TAG is identified to include a primaryserving cell (PSC) when the TAG index field is set to
 00. 9. The methodof claim 6, wherein the MAC CE is identified to include the TAC fieldwhen the index indicated by the LCID is set to
 11101. 10. The method ofclaim 6, further comprising: receiving, from the BS, a RRC message, theRRC message including a TAG configuration information which includes atleast one serving cell corresponding to at least one TAG to beconfigured.
 11. A base station (BS) performing an uplink synchronizationin a wireless communication system, the BS comprising: a medium accesscontrol (MAC) processing unit configured for: generating a timingadvance command (TAC) field with 6 bits indicating a timing alignmentvalue (TAV) and a timing alignment group (TAG) index field with 2 bitsidentifying a TAG to control uplink timing adjustment identically withthe TAV; generating a MAC control element (CE) by including the TACfield and the TAG index field in the MAC CE; setting a logical channelidentifier (LCID) indicating an index to identify whether the MAC CE isfor TAC; generating a header by including a subheader with the LCID inthe header; and generating a MAC protocol data unit (PDU) comprising theheader and the MAC CE; and a transmitting unit configured fortransmitting the MAC PDU to a user equipment (UE).
 12. The BS of claim11, wherein the MAC processing unit is further configured for generatingthe TAG index field with 2 bits identifying the TAG among 4 TAGs definedby the 2 bits.
 13. The BS of claim 11, wherein the MAC processing unitis further configured for setting the TAG index field to 00 to include aprimary serving cell (PSC) in the TAG.
 14. The BS of claim 11, whereinthe MAC processing unit is configured for setting the index indicated bythe LCID to 11101 to represent that the MAC CE includes the TAC field.15. The BS of claim 11, further comprising: a radio resource control(RRC) processing unit configured for generating a RRC message, the RRCmessage including a TAG configuration information which includes atleast one serving cell corresponding to at least one TAG to beconfigured, wherein the transmitting unit is further configured fortransmitting the RRC message to the UE.
 16. A method of performing anuplink synchronization by a base station (BS) in a wirelesscommunication system, the method comprising: generating a timing advancecommand (TAC) field with 6 bits indicating a timing alignment value(TAV) and a timing alignment group (TAG) index field with 2 bitsidentifying a TAG to control uplink timing adjustment identically withthe TAV; generating a MAC control element (CE) by including the TACfield and the TAG index field in the MAC CE; setting a logical channelidentifier (LCID) indicating an index to identify whether the MAC CE isfor TAC; generating a header by including a subheader with the LCID inthe header; generating a MAC protocol data unit (PDU) comprising theheader and the MAC CE; and transmitting the MAC PDU to a user equipment(UE).
 17. The method of claim 16, wherein the TAG is identified among 4TAGs defined by the 2 bits.
 18. The method of claim 16, wherein the TAGis identified to include a primary serving cell (PSC) when the TAG indexfield is set to
 00. 19. The method of claim 16, wherein the MAC CE isidentified to include the TAC field when the index indicated by the LCIDis set to
 11101. 20. The method of claim 16, further comprising:transmitting, to the US, a RRC message, the RRC message including a TAGconfiguration information which includes at least one serving cellcorresponding to at least one TAG to be configured.